Can a late N topdressing help wheat yields this year?

With the wheat being unusually late and small this year, and still pale or yellowish in some cases, producers may be wondering whether they would still get any benefit from topdressing with nitrogen (N) at this point in the season.

Reasons for pale, yellowish wheat at this point in the year

If a topdressing application of N had been made earlier and the wheat is still short and pale, the cause is most likely because: (1) root development is poor due to dry conditions, limiting total nutrient uptake, (2) the applied N hasn’t yet been moved into the soil by precipitation, (3) only a small portion of the N in the soil has been moved to the surface of the plant roots (which have been stunted by the dry conditions) with water via mass flow, the primary N uptake process, (4) the N has been tied up by surface residue or lost due to volatilization, or (5) another nutrient, such as sulfur, may be limiting.

With a rain, most of these issues may be resolved. In many of these dry soils, the breakdown of crop residues and organic matter will kick into high gear once it rains, releasing N and S. Root growth will also kick into high gear and expand the exploited zone of soil. As the water begins to flow towards the roots, mobile nutrients like nitrate and sulfate dissolved in that soil water and will flow to the plant roots for uptake. But if the crop wasn’t topdressed, and soil N supplies are deficient, or if a rain does not look likely, producers may wonder if an application of N or S now would help the yield potential of pale or yellowish N-deficient wheat.

When yield components of wheat are determined

Keep in mind some of the basics of wheat physiology and yield when considering late fertilization. There are three primary components of yield: the number of heads per foot of row; the size of the head or number of kernels per head; and the size of the individual kernels. Number of heads is a function of the initial plant stand, tillering, and tiller survival.

In many Kansas fields initial stands were spotty, but tillering was adequate. As the dry spring progressed, tillers began to abort due to drought stress. Head size is determined around Feekes 5, prior to first hollow stem. The plant responds to environmental conditions and produces a head of the size that it can successfully fill under existing conditions. If conditions are bad, which was the case in many fields, small heads are formed, and the plant sheds tillers through tiller abortion, further reducing total potential kernel numbers. So by jointing and stem elongation, the maximum number of kernels which can be produced is already set.

The final yield component, kernel size, is yet to be determined at this point in the season. Weather conditions at heading and pollination will impact the number of actual kernels set per head, and rain during grain fill will produce big plump kernels. While kernel size is very important and will determine the number of kernels actually set which can be harvested and marketed, maximum kernel size is set genetically.

Unfortunately we can’t produce a wheat kernel the size of an apple. So while correcting an N or S deficiency now, as the crop is approaching or at heading, can have some potential to preserve potential yield or increase yield by maximizing the size and number of potential kernels harvested, the extent of that response is limited since the maximum number of heads and kernels per head was determined several weeks ago.

Leaf burn considerations

The potential for leaf burn is an important consideration with late applications of N. Traditional broadcast applications of 28% liquid N with flood or flat fan nozzles can cause foliar burn to the flag leaf, especially at high air temperatures. Leaf burn is a serious concern when it occurs on the upper leaves, since the wheat relies on these leaves for grain fill.

Once the head begins to emerge, the risk of injury from leaf burn from broadcast UAN far exceeds any potential response to additional fertilizer, even if the wheat is N deficient. At that stage of growth, a foliar N product is much safer to the crop.

To reduce the potential for leaf burn, there are alternative ways to apply traditional liquid N sources other than the standard spray nozzle. Streamer bars, a 10- to 15-inch long plastic bar which can be used with traditional spray booms in place of the nozzle, provide a solid stream of liquid fertilizer spaced every 5-6 inches. These streams of liquid greatly reduce foliar burn as compared to complete foliage coverage with standard flat fan spray nozzle. Broadcast granular urea also produces limited leaf burn as compared to sprayed UAN.

Foliar nitrogen products

Various foliar nitrogen (N) fertilizer products are available. These products range in analysis and can include straight nitrogen products or mixtures of N plus other macro and micro nutrients. The straight nitrogen products will typically have an analysis similar to traditional liquid N fertilizers, such as 25 to 30 percent N.

One of the main differences between traditional UAN and the foliar products is that a certain percentage of the N in the foliar fertilizers is commonly in some type of slow-release form. As a result, these specialty products are generally safer for application directly to the foliage in later stages of growth and result in less leaf burn than traditional UAN products.

K-State has tested many different types of foliar N fertilizer products over the years. Foliar N fertilizer products are just as effective as traditional N fertilizers on a pound-for-pound basis, but they are not more effective than traditional N fertilizers. They can be applied in a broadcast spray application at later growth stages of wheat growth than traditional N fertilizer products without damaging the wheat.

One of the reasons the foliar products have not been found to be more effective than traditional soil application is that only a small portion of the N applied as a foliar application to wheat actually moves into the plant through the leaf tissue. An excellent study done in Canada a few years ago found that when care was taken to prevent foliar applied N from reaching the soil, only 8-12% of the applied N was recovered by the plant, compared to 35 to 70% of soil applied N being taken up by the plant. Thus it is very likely that many foliar applied fertilizers are actually taken up through the roots once they wash off the plant.

Invariably, the foliar products will be higher in terms of cost-per-pound-of-N than the traditional N fertilizers. In unusual situations (well after jointing or when trying to increase protein levels), the foliar N products would have some premium value since traditional N products could not safely be used in a broadcast spray application.

The bottom line

The bottom line is, there are limited opportunities to increase wheat yields in most fields this late from N applications.

The last opportunity for improving yield is maximizing grain fill with larger berries. However, current research by K-State has shown that flag leaf emergence is the last growth stage that has marginal reliability for getting yield and protein responses to N applications in Kansas.

Foliar N products can be used for later applications, but the limited amounts of N which can be applied based on the labels of many of these foliar products limits their use in situations where large amounts of N are needed, and the potential for yield increase is limited physiologically.

The potential for yield response to an N application at this point in the season is very low. Additional N applications now will likely result in reduced profit per acre.